The present invention generally relates to intersystem coordination for maintenance of communication and more specifically relates to the method and apparatus useful for enabling a physically mobile remote unit to be able to automatically transmit and receive messages as it travels from the radio coverage area of one radio communications system to the radio coverage area of another, independent, radio communications system.
Radio communication services often grow in geographic extent such that a communication service radio coverage area offered by one coordinated system expands to reach if not partially cover areas which are already covered by a second coordinated but independent system. Typically, a mobile or portable radio transceiver (a remote unit) associated with one system would be denied service on the second system even though the radio frequencies in use may be compatible. Proposals have been advanced to enable a remote unit to be able to obtain service in a "foreign" system but generally such systems require the remote unit operator or his fixed station cocommunicator (dispatcher) to manually take some action to preregister in the foreign system. (See "Advanced 800 MHz Trunked Radio Systems" published by Motorola, Inc. as document RO-6-07 on November 1982, page 4).
Manual registration, besides being inconvenient for the user, forces the user to learn the radio boundaries of the systems so that the user must register in system A while in certain locations and in system B while in other locations. This boundary learning detracts from the user's main purpose for having a remote unit: to communicate his message. Furthermore, registration in one system may result in messages directed to the user in the other system to be lost because the user is no longer present there.
Automatic handoff of radio communications has been well developed in cellular radiotelephone systems and the equipment and processes necessary to the handoff are well known. Automatic handoff, however, occurs within a single coordinated system and not between separate systems. The body of knowledge regarding intrasystem handoff, therefore, is not of particular value when coordination between two independent systems is minimal.